This invention relates generally to golf balls, and specifically to ionomer covered golf balls (more preferably, soft ionomer covered golf balls) having improved heat resistance. These golf balls exhibit improved melt resistance and heat stability when subject to high temperatures, i.e. 50xc2x0 C. or more. Such high temperatures can be present in the trunks and/or interiors of automobiles, shipping containers, truck trailers, warehouses, etc., in warm climates and/or on hot summer days. The golf balls of the invention exhibit improved heat stability without sacrificing properties such as distance, durability and/or playability.
Before the development of ionomers, balata was the preferred material of formulations for golf ball covers. Polyethylene was also proposed for use as a golf ball cover material, but was generally deemed highly inferior to balata in imparting playability and durability characteristics to the ball due to its brittleness and high hardness, and thus never became a commercially successful golf ball cover material.
Balata golf ball covers have now been replaced to a great extent by ionomeric cover materials. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E.I. DuPont deNemours and Company (see U.S. Pat. No. 4,884,814) under the trademark SURLYN(copyright) and by the ExxonMobil Corporation (see U.S. Pat. No. 4,911,451) under the trademarks ESCOR(copyright) and IOTEK(copyright), have become the materials of choice for the construction of golf ball covers over the traditional xe2x80x9cbalataxe2x80x9d (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the overall durability necessary for repetitive play.
Ionomeric resins are generally ionic copolymers or terpolymers of an olefin such as ethylene and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. Optionally, an acrylate can also be present. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, such as improved durability, for golf ball cover construction over balata.
In this regard, the metal ions serve as crosslinking agents, as they are ionically bonded to carboxylic acid groups in adjacent copolymer chains. However, instead of having a thermally irreversible covalent bonding crosslinking, ionomers have a thermolabile crosslinking, where metal ions become part of the chemical structure of the ionomer upon crosslinking so that these crosslinks are reversible. Consequently, ionomers are subject to distortion or degradation at high temperatures.
Furthermore, the advantages gained through the use of ionomer resins in golf ball cover formulations in increased durability have been offset to some degree by the decrease in playability. Although ionomeric resins are very durable, they tend to be very hard when utilized for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in flight.
As a result, while there are currently numerous commercial grades of ionomers available with a wide range of properties that vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the improved impact resistance and carrying distance properties produced by the xe2x80x9chardxe2x80x9d ionomeric resins, but also the playability (i.e. xe2x80x9cspinxe2x80x9d) characteristics previously associated with the xe2x80x9csoftxe2x80x9d balata covers, properties which are still desired by the more skilled golfer.
In various attempts to produce such an ideal golf ball, the golfing industry has blended hard ionomeric resins with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of hard ionomer resins with softer polymeric materials have generally been unsatisfactory, as these balls exhibit numerous processing problems. In addition, the balls produced by such a combination usually lack the distance desired by a golf ball.
In addition, various xe2x80x9chard-soft ionomeric blendsxe2x80x9d, i.e. mixtures of ionomer resins which are significantly different in hardness and/or flexural modulus, have been attempted. However, until the development of the specific blend combination set forth in U.S. Pat. No. 4,884,814, directed to low modulus golf ball cover compositions, these balls were not particularly commercially viable. In this regard, although the balls produced using the hard-soft ionomer blends exhibited enhanced playability characteristics, they lacked the durability needed for continuous play.
Moreover, while there are numerous advantages to the use of ionomers in making golf ball covers, one drawback of conventional golf balls with ionomeric covers are that the covers are prone to softening at temperatures of approximately 50xc2x0 C. or above. As a result, the ionomeric covers (and in particular, soft ionomeric covers) may lose their dimple pattern or develop flat spots if exposed to high temperatures.
Accordingly, it would be useful to develop a golf ball with an ionomeric cover which is highly resistant to high temperature distortion or degradation without sacrificing the properties of distance, durability and/or playability.
Furthermore, it would be useful to improve the heat resistance of soft ionomeric golf ball covers without substantially hardening the covers. This would result in the production of a soft ionomer covered golf ball having the distance, durability and/or playability characteristics desired while also being resistant to degradation at high temperatures.
The present invention relates to new and improved golf balls which overcome the above referenced problems and others. In this regard, the present invention is directed to golf balls having improved heat and/or melt resistance. This enables the golf balls to withstand prolonged exposure to heat during use or storage.
In one aspect, the present invention is directed to a golf ball comprising a core and a cover disposed about the core. The cover comprises less than about 20 parts by weight of at least one hard ionomer copolymer and at least about 80 parts by weight of at least one soft ionomer. The at least one hard ionomer exhibits a Vicat softening temperature of at least about 74xc2x0 C. The at least one soft ionomer exhibits a Vicat softening temperature of less than about 50xc2x0 C.
In another aspect, the present invention is directed to a golf ball comprising a core and a cover disposed about the core. The cover comprises less than about 20 parts by weight of at least one hard ionomer copolymer and at least about 80 parts by weight of a blend of two soft ionomer terpolymers. The at least one hard ionomer copolymer exhibits a Vicat softening temperature of at least about 74xc2x0 C. The blend of two soft ionomer terpolymers exhibit a Vicat softening temperature of less than 50xc2x0 C.
In a further aspect, the present invention is directed to a golf ball comprising a core and a cover disposed about the core. The cover comprises from about 1 to about 50 parts by weight of at least one non-ionomeric terpolymer and from about 99 to about 50 parts by weight of at least one soft terpolymer. The non-ionomeric terpolymer exhibits a Vicat softening temperature of at least 74xc2x0 C. The at least one soft ionomer terpolymer exhibits a Vicat softening temperature of less than 50xc2x0 C.
Another object of the invention is to provide a method for improving the heat and/or melt resistance of a golf ball cover. The invention is directed to any type of ionomer covered golf ball including wound, two-piece, three-piece and multi-layered golf balls.
Yet another object of the invention is to provide a golf ball with a soft ionomer cover which is well-suited for repetitive play and exhibits improved heat and/or melt resistance when subject to high temperatures, i.e. 50xc2x0 C. or more.
Another object of the invention is to provide a golf ball with a very soft ionomeric cover having enhanced heat stability and/or improved heat and melt resistance. These golf balls also exhibit the feel and playability characteristics that highly-skilled golfers prefer. This enables the soft covered golf balls to exhibit enhanced dimple retention during prolonged exposure to high temperatures.
Other objects will be in part obvious and in part pointed out in more detail hereinafter.
In accordance with the present invention, there is provided a golf ball comprising a core and a dimpled cover having a Shore D hardness of 63 or less as measured on a non-dimpled portion of the cover, the dimples maintaining their shape when the cover is subjected to prolonged heat exposure at 160-180xc2x0 F. (71-82xc2x0 C.) for at least one hour.
The core component of the invention can consist of a solid or wound core. Additionally, the core can consist of one or more layers. Similarly, the cover component of the golf ball can consist of one or more layers. However, the outer layer of the golf ball is comprised of an ionomer based material.
More particularly, the outer cover is a blend of one or more ionomer copolymers and/or terpolymers and one or more ionomers having a high Vicat softening temperature. Preferably, the high Vicat softening temperature ionomer also has a high melt temperature. More preferably, the difference between the high melt temperature and the Vicat softening temperature of the high melt ionomer is minimized. It has been found that such high melt ionomers act as good heat stability modifiers for ionomer covers, and in particular, for soft ionomer covered golf balls.
Along these lines, the Vicat softening temperature of the high melt ionomer incorporated into the present invention is 74xc2x0 C. or more, preferably 80xc2x0 C. or more, and most preferably, 84xc2x0 C. or more. The melt temperature of the high melt ionomer is 96xc2x0 C. or more, preferably 98xc2x0 C. or more, and most preferably, 100xc2x0 C. or more. Furthermore, the high melt ionomer utilized in the invention exhibits a difference in melt temperature and Vicat temperature of 25xc2x0 C. or less, more preferably 19xc2x0 C. or less, most preferably 17xc2x0 C. or less.
Additionally, the high melt ionomer of the present invention can also be utilized to formulate an inner cover layer or mantle of a multi-layer golf ball. Accordingly, one or more of the ionomer layers of a multi-layer golf ball can exhibit high heat stability.
The dimpled cover golf ball of the present invention can have a Shore D hardness cover of 63 or less, preferably 55 or less, and most preferably 50 or less. The dimpled cover is formed from a cover material which comprises at least 80 parts by weight of copolymer or terpolymer ionomer (preferably 80-97, most preferably 91-94) and 3-20 parts by weight of high melt ionomer (preferably 3-10, most preferably 6-9).
These and other objects and features of the invention will be apparent from the following descriptions and from the claims.